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Distillation
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Introduction
This report examines the distillation process. This will enable the reader to understand the
necessary components along with distillation calculations. Distillation is a process that
separates two or more components into an overhead distillate and bottoms. The bottoms
product is almost exclusively liquid, while the distillate may be liquid or a vapor or both.
The separation process requires three things. First, a second phase must be formed so that
both liquid and vapor phases are present and can contact each other on each stage within
a separation column. Secondly, the components have different volatilities so that they
will partition between the two phases to different extent. Lastly, the two phases can be
separated by gravity or other mechanical means. Distillation differs from absorption and
stripping in that the second phase is created by thermal means (Seader, 1998).
Assumptions
The following assumptions are implied when using this method (McCabe,
1993):
a. Constant Molal Overflow. The molar flow rates of the vapor and
liquid are nearly constant in each section of the column. This also
ensures the operating lines are straight lines.
b. Heat Effects are negligible. For example, heat losses to and from
the column are small and neglected.
c. For every mole of vapor condensed, another mole of liquid is
vaporized.
d. The liquid and vapor leaving the tray is in equilibrium with the
vapor and liquid entering the tray.
Procedure
If an equilibrium curve is not given, draw a y-x diagram (y representing the vapor
phase and x the liquid). The equilibrium curve can be obtained by relating the
relative volatility to the composition of the liquid:
y = a*x/(1+x(a-1)) (4)
This shows the bubble-point and dew point of a binary mixture at constant
pressure. An equilibrium line describes the compositions of the liquid and vapor
in equilibrium at a fixed pressure. The equilibrium line crossing the forty-five
degree line is an indication of an azeotropic mixture. An azeotrope is a liquid
mixture which when vaporized produces the same composition as the liquid. If
the mixture is azeotropic, then more advanced types of separation must be
considered
Choose Actual Reflux Ratio:
As the reflux ratio increases, the number of trays and thus the capital cost of the column
decreases. However, as a trade-off, an increase in reflux ratio will also increase the vapor
rate within the tower, thus increasing expenses such as condensers and reboilers (4).
Most columns are designed to operate between 1.2 and 1.5 times the minimum reflux
ratio because this is approximately the region of minimum operating cost. Therefore,
based on first estimates, the operating reflux ratio is equated so that (Douglas, 1988):
Choose Theoretical Reflux Ratio
As the reflux ratio increases, the number of trays and thus the capital cost of the column
decreases. However, as a trade-off, an increase in reflux ratio will also increase the vapor
rate within the tower, thus increasing expenses such as condensers and reboilers
(Douglas, 1988). Most columns are designed to operate between 1.2 and 1.5 times the
minimum reflux ratio because this is approximately the region of minimum operating
cost. Therefore, based on first estimates, the operating reflux ratio for the analytically
method is equated so that (Douglas, 1988):
Alternatives
Extraction: A process where one or more solutes are removed from a liquid by
transferring the solute(s) into a second liquid phase. Liquids must be immiscible or
partially immiscible. Vaporization is not required, thus extraction can be done at low
temperatures and is a gentle process suitable for unstable molecules (Wankat, 1988).
· Adsorption: Involves the transfer and equilibrium distribution of one or more solutes
between a fluid phase and particles. Liquid mixture is brought into contact with a
microporous solid allowing adsorption of certain components in the mixture takes
place on the internal surface of the solid. The adsorbent (solid) is insoluble in the
liquid, and the components being adsorbed are called solutes (Perry’s, 1997).
· Condensation: Occurs when a saturated vapor comes in contact with a surface whose
temperature is below the saturation temperature. Film-type condensation occurs
when a film layer of condensate is formed on the surface. Dropwise condensation
occurs when appears in many small droplets at various points on the surface (Perry’s,
1997).
Reactive Distillation: Works by adding an entrainer that reacts with one component in
a mixture that is difficult to separate if relative volatilities of two components are very
close (less than 1.1). In the first column, the entrainer will react with one component
to increase the relative volatility so that it may be separated while in a second column,
the reaction can be reversed so that the entrainer can be recycled (King, 1980).
· Extractive Distillation: This operation requires two distillation columns. A heavy
nonvolatile component is added near the top of the tower to modify the activity
coefficients between two components. A pure component in the overhead in the first
column is obtained while the other component is recovered in the overhead in the
second column (King, 1980).